1. Field of the Invention
The present invention pertains to the simulation of gravitational forces on a living organism. More specifically, the invention provides a method and apparatus for simulating such forces on the circulatory system, typically of a human person, and particularly in the area located below the heart in an ordinary standing position. It can be readily appreciated that ordinary gravitational forces would, in the absence of some compensating mechanism, cause blood to accumulate or "pool" in the lower body, and especially the legs, of a standing human or other organism. To offset this tendency in the human, a number of anatomical and physiological mechanisms have evolved which either tend to resist downward shifts of fluid in those portions of the body and/or to enhance or assist upward flow. For example, the large veins in the legs of a human are surrounded with powerful muscles which tend to squeeze the enclosed vein and thereby urge the blood upwardly through a series of one way valves in those veins. Because the potential pooling problem is increased with distance from the heart, these mechanisms become more prevalent in the lower parts of the body. For example, the aforementioned one way valves in the vein increase in frequency toward the feet of a human being. Likewise, various muscles associated with the superficial veins become thicker with increasing distance from the heart to compensate for correspondingly increasing hydrostatic pressures.
The human person or other organism may be placed in situations in which the ordinary gravitational forces, which are counteracted by the aforementioned anatomical and physiological mechanisms, are not exerted. For example, if a person is bedridden or otherwise positioned with the legs outstretched generally horizontally, the tendency for gravity to cause pooling of the blood in the feet is virtually eliminated. Likewise, when a person leaves the earth's atmosphere, as in space flight, the "weightlessness" which is experienced is, per se, an absence of the ordinary gravitational forces, and occurs regardless of the position of the person.
Accordingly, it may be desirable to simulate ordinary gravitational forces on such persons. For example, when these gravitational forces are absent for an extended period of time, e.g. during space flights or a lengthy illness, several effects occur which in turn inhibit the resumption of normal functioning of the aforementioned mechanisms for resisting blood pooling. For example, after a period of weightlessness, the body will have readjusted the quantity and distribution of blood in the circulatory system, so that there will be a relatively low volume of blood in general accompanied by abnormally low concentrations of blood in the lower trunk, legs and feet. Then, when the earth's atmosphere is reentered, the body must attempt to return to a normal distribution of blood throughout the lower body area, with overall reserves depleted. This subjects the astronaut to physical stress at a time when other demands are high and such stress can least be tolerated. Even more seriously, where a person has remained bedridden for an extremely long period of time, or should an astronaut remain in space for a comparable length of time, the body's aforementioned anatomical and physiological gravity-counteracting mechanisms may atrophy through nonuse.
It is therefore desirable to be able to simulate the normal gravitational forces while the person is bedridden or in a weightless condition to eliminate such problems. In still other instances, it may be desirable to simulate normal gravitational forces, either on earth or in space, for scientific or experimental purposes.
2. Description of the Prior Art
Several prior devices are known for simulating gravitational forces by applying a negative pressure to the lower portion of the body of a person. One such device incorporated a rigid, stationary cabinet into which the person would insert his lower body. It was then necessary to remain in the cabinet, effectively immobilized, for a suitable length of time for application of the negative pressure. Another device, developed in the U.S.S.R., was "portable" in that it included a somewhat trouser-like garment permitting at least some movement by the user while the negative pressure was being applied.
Aside from their cumbersome nature and undue complexity, both of these types of devices suffer from a major problem in that they apply a single, constant value negative pressure to the entire portion of the body being treated, e.g. from the chest or waist to the feet. This is not only a highly inaccurate simulation of normal gravitational forces, which produce a gradient of hydrostatic pressures from the heart to the feet of a standing person, but introduces other related problems. For example, in order to avoid subjecting the lower trunk of the person to a harmfully low pressure, it was necessary to select a pressure value which was not low enough to be suitably beneficial to the area of the lower limbs, ankles and feet. Even so, it was necessary to attempt to provide extremely accurate pressure value controls, and such controls had to be meticulously monitored to ensure against cardiovascular disturbances. Furthermore, even when selected at a safe level, e.g. -50 mm Hg, such a constant pressure value, representing a compromise between appropriate values for the trunk area and foot area, represented a relatively large pressure differential as compared to the atmospheric pressure existing outside the body enclosure. If then became difficult to adequately seal the enclosure with respect to the person's body, without causing discomfort or undue constriction of the body, and also necessitated relatively high capacity pumps for applying and maintaining the negative pressure.
A number of other prior devices are diametrically opposed to the present invention, in both purpose and operation. Examples of such devices are disclosed in U.S. Pat. No. 3,548,809; No. 2,531,074; No. 4,029,087; No. 2,495,316; No. 3,574,236; No. 3,659,593; No. 3,862,629; No. 4,066,084; and No. 4,270,527. Each of the devices disclosed in these patents is intended not to simulate ordinary gravitational forces, but rather, to simulate or enhance the counteractive mechanisms of the body. Typically, these devices apply a positive pressure or compressive force to the body, sometimes with a pulsing, rippling or massage-like action. Furthermore, the devices are, in general, incapable of applying negative pressures to a portion of a body for several reasons. In the first place, the bladders or the like to which the positive pressures are applied would simply collapse if subjected to negative pressures. Furthermore, these bladders are not in effective direct fluid communication with the person's body, but rather represent separate sealed compartments which may be inflated so that their exterior surfaces abut the body to indirectly transmit compressive forces only. Thus, even if negative pressures could somehow be applied to these bladders without their collapse, these negative pressures would not be communicated to the adjacent leg or other body portion.
Still another prior technique for simulating gravitational forces involved increasing the internal blood pressure of the organism, as by applying carefully controlled compressive loads to the thoracic area, and then counteracting this increased internal pressure with positive pressures applied externally to the legs. While this system produces a very accurate simulation of normal gravitational forces on a standing organism, it is complicated, expensive, potentially dangerous, and generally inappropriate for uses other than scientific experimentation.